The present invention relates to a switched-mode power supply having a switching transistor, a transformer having a primary winding, which is connected in series with the switching transistor, and at least one secondary winding, a starting circuit, a driver circuit for driving the switching transistor, and a threshold value circuit.
Switched-mode power supplies of this type require a starting circuit through which, after the switching-on of the switched-mode power supply, a current flows for the purpose of supplying the driver stage, since otherwise the switched-mode power supply does not start oscillating. This current must also be sufficiently large to activate transistors of the driver stage and to supply the switching transistor with a sufficiently large control voltage or a control current so that the said transistor is activated. When the switching transistor is activated, a first current flows through the primary winding of the transformer, as a result of which a voltage is produced across a secondary winding, for the purpose of supplying the driver circuit. In the course of further operation, the switched-mode power supply supplies itself with power to a sufficient extent via this winding, with the result that the starting circuit is no longer required.
However, since the starting circuit is continually in operation and consumes power of the order of magnitude of one watt or more, this is highly undesirable particularly in the case of switched-mode power supplies which have a standby mode or are connected for a long time or permanently.
In order to reduce the power loss, switched-mode power supplies of various designs are known, some of which are complicated, however, as specified in DE 196 52 604, or require a starting oscillator, as described in EP 0 701 318 A1. Other designs for reducing the standby power loss, such as supplementary devices for switching off a television set, described in EP 0 804 026 A2, for example, entail disadvantages for a user under certain circumstances, since they do not react selectively to remote-control signals.
WO92/17934 discloses a starting circuit for a switched-mode power supply, which starting circuit has an integrated circuit with a Zener diode through which delayed switching-on of the integrated circuit is effected. During the delay phase, the integrated circuit is initialized and the parameters and components are brought to a defined state. At the end of the initialization phase, the integrated circuit is changed over to steady-state normal pulse operation.
The object of the present invention is to specify a switched-mode power supply of the type mentioned in the introduction in which the power consumption of the starting circuit can be kept as low as possible.